Sintered anisotropic alnico magnet



Patented Mar. 20, 1951 .SINTEKED ANISOTROPIC .ALNICO .MAGNET Robert J.Studders, Schenectady, 1 rassignor to General Electric Company, acorporation of New York No Drawing. Application April 13, 1948, SerialNo. 20,842

B GIaimS. 1

The present invention relates to sinteredianisotropic lk-lnico magnetalloys. It is particularly concerned with sinterel Alnico magnetscontaining zirconium, which magnets are prepared by powder metallurgytechniques and have BHmax values corresponding to those, or approachingthose of the best cast Alnico magnets. Essentially isotropic permanentmagnets of the type known as Alnico magnets contain iron, nickel andaluminum as basic or essential ingredients. It is well known or has beenproposed to include other elements such as cobalt, copper, titanium,silicon, chromium, molybdenum, tungsten, manganese, sulphur, and otherseither for the purpose of modifying or enhancing the physical and/ormagnetic properties of the alloys. Many of these magnetic alloys havebeen produced in bothcast and sintered form, and prowided the properprocedures are followed in the manufacture of the sintered products toprevent oxidation of the aluminum and any other highly oxidizableelements, it has been found that for most of the alloys the sinteredproducts exhibited the same or approximately the same magneticproperties as the cast products of the same composition.

Another known development in the Alnico mag- "net field was thediscovery described more fully in Jonzas'iPatent 2295;082 that certaincast nickelal uminum-iron alloys having a cobalt content of 16 to 30 percent, a nickel content of 12 to 20 per cent, and an aluminum content of6 to 1.1 per cent Whenheat treated in a magnetic field would exhibit'BHmx values in one direction (anisotrope) which was at least 50 percent, and generally over 100 per cent, higher than that for the samealloy whose magnetic properties were substantially equal in alldirections (isotrope). As a result of this development there is nowcommercially available cast Alnico magnets having approximately thefollowing minimum magnetic properties:

Peak H oersteds 2,000 Peak B gauss 15,700 Coercive force He oersteds5'75 Residual Br gauss 12,000 BHmax gauss-oersteds 4=.5 10

.A cast magnet possessing these excellent magnetic values consists of analloy of about 8 per cent aluminum, 14 per cent nickel, 24 per centcobalt, 3 per cent copper, balance substantially iron, the exceptionalmagnetic properties being obtained by heat treatment or coolin of thealloy in a magnetic field as described in the aboveor m) mentioned Jonaspatent. Because of the exceptional magnetic qualities of this cast alloyand the known advantages of the powder metallurgy techniques numerousattempts have been made to duplicate its magnetic properties in a.:sintered magnet alloy.

Unlike the isotropic type of Alnico alloys, it was found that simpleduplication of the chemical composition of the anisotropic type ofalloy, as :described in the Jonas patent, in a :sintered product, didnot result in a very satisfactory permanent magnet. There appears to bemany other factors which influence the efiectiveness of the magnet cfield treatment in imparting superior available energy products to thealloy.

'In my 'copending application, Serial No. 9,013, filed "February 17,1948, and assigned to the same assi-gnee as the present invention, thereare described anisotropic sintered magnets possessing high BHmax values,the sintered magnets containing titanium as an essential ingredient.

The present invention is based on the discovery that an anisotropics'intered magnet can also be obtained by including additions ofzirconium in the com-positions employed in making cast ani otropicmagnets. The zirconium appears to perform the same function in thesintered alloys as does the titanium although, in general, only abouthalf as much by weight of zirconium is re quired to obtain the samemagnetic properties in the finished sintered alloy.

The presence of zirconium in the sintered products appears to make themagnetic field treatment more efiec'tive in obtaining a high availableenergy product, which product is an accepted criterion for comparingpermanent magnet alloys. In addition, by using the zirconium in smalleramounts by weight than titanium to obtain a product having equivalentmagnetic characteristics, it has been found that there is less tendencytoward surface oxidation and distort-ion of the sintered material.

The sintered magnets of the present invention are generallycharacterized by a zirconium content of at least .15 to about 1.25 percent, generally from 0.3 to 1.0 per cent. While the invention is broadlyapplicable to the Various known anisotropic magnet alloys characterizedby a cobalt content of 16 to 30 per cent, a nickel content of 11 to 20per cent, and an aluminum content of 6 to 11 per cent, the preferredsintered magnets are those containing 7.5 to 9 per cent aluminum, 0.3 to0.8 per cent zirconium, 22 to 26 per cent cobalt, 2 to 4 per centcopper, 13 to 15 per cent nickel, remainder substantially all ironexcept for incidental impurities. Small amounts of titanium may also bepresent in the sintered alloy in amounts not exceeding about 1.25 percent, and when titanium is present it may function as a substitute for apart of the zirconium in the proportions of about 2 parts titanium foreach part zirconium.

Particularly good results have been obtained by employing 8.5 per centaluminum, 0.5 per cent zirconium, 25 per cent cobalt, 3.25 per centcopper, 14 per cent nickel, balance iron. The sintered products areprepared in the manner usually employed in making sintered Alnicomagnets as described, for example, in Howe Patents 2,192,743 and2,192,744. The source of zirconium may suitably be a foundation alloy ofnickel-zirconium such as 50-50 nickel-zirconium alloy powder, while thefoundation alloy of iron-aluminum, cobalt-aluminum, or nickel-aluminumcan be used as a source of aluminum. The finely divided materials aremixed in the desired proportions, and pressed to the desired form orshape. The pressed products are sintered in a hydrogen atmosphere attemperatures of from 1000 to 1400 C. preferably at a temperature belowbut close to the melting point of the alloy. The time required for thesintering action will, of course, depend upon the furnace load and sizeof the pieces to be sintered. The sintered material can then benormalized by heating to an elevated temperature after which thesintered product is subjected to a further heat treatment in a magneticfield as described in the Jonas Patent 2,295,082 to make the sinteredalloy magnetically anisotropic. The heat treatment in the magnetic fieldis preferably carried out by withdrawing the sintered alloy compactsfrom the sintering zone of the furnace at a temperature of about 1250 C.and controlling their cooling cycle in a magnetic field of proper fieldstrength. Further low temperature treatments may be applied as describedby Jonas.

The resultant magnets usually exhibit a BHmax at least equal to 3.5 X 10gauss-oersteds, a Br of at least 10,000 and Ho of at least 600. Typicalmagnetic properties of a sintered magnet having the preferredcomposition set forth hereinbefore include a Br of 10,600 gauss an E0 of600 cers eds, and a BHmax of 4x10 These BHmax values are to be comparedwith values of 1.4 to

10x10 for the best commercially available sintered Alnico magnets and4.5 to 6 10 for the best anisotropic cast Alnicos. The effect of thezirconium on the magnetic properties of the sintered material becomesclearly evident from a comparison of the magnetic characteristics of aheat-treated, sintered mixture of from 8 to 8.5 per cent aluminium, 24per cent cobalt, 14 to 14.5 per cent nickel, 3.25 per cent copper,balance iron with the magnetic characteristics of a similarly preparedand heat treated sintered material of the same composition butcontaining in addition .5 per cent zirconium. The sintered alloy free ofzirconium is characterized by a Br of 11,000 to 12,000 and He of 450 to550 and BHmax of 2.0 to 3.0 10 whereas the sintered alloy containing anaddition of .5 per cent zirconium has a slightly lower Br of 10,600, asomewhat higher He of 600, and a much higher BHmax of 4x10".

What I claim as new and desire to secure 'by Letters Patent of theUnited States, is:

1. An anisotropic sintered permanent magnet containing 7.5 to 9 per centaluminum, 13 to 15 per cent nickel, 22 to 26 per cent cobalt, 2 to 4 percent copper, 0.3 to 1.0 per cent zirconium, balance iron except forincidental impurities, said magnet having a BHmax in the principaldirection at least equal to 3.5 10 I,

2. An anisotropic sintered permanent magnet consisting of 8.5 per centaluminum, 14 per cent nickel, 25 per cent cobalt, 3 per cent copper, 0.3to 0.8 per cent zirconium, balance iron except for incidentalimpurities, said sintered magnet having a BHmX of at least 3.5 10

3. An anisotropic sintered permanent magnet consisting of 8.5 per centaluminum, 14 per cent nickel, 25 per cent cobalt, 3 per cent copper, 0.5per cent zirconium, balance substantially all iron, said magnet having aBrim; of at least 3.5 10

ROBERT J. STUDDERS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,096,670 Catherall Oct. 19, 19372,185,464 Howell Jan. 2, 1940 2,192,744 Howe Mar. 5, 1940 2,245,477Jonas June 10, 1941 2,285,406 Bieber June 9, 1942 2,295,082 Jonas Sept.8, 1942 2,384,450 Bieber Sept. 11, 1945 FOREIGN PATENTS Number Country 7Date 439,543 Great Britain Dec. 9, 1935 522,731 Great Britain June 26,1940

